They built
a 25-centimeter (15 inch) robotic wing, which flaps and rotates at one-hundredth
a fly's speed in a two-ton tank of mineral oil. Three motors move the
robotic wing back and forth in precise motions determined by a computer.
Bubbles pumped into the tank show the aerodynamic patterns. Sensors measure
the forces on the wings during each phase of the stroke.

video:
"Robofly"

animation:
forces on Robofly's wing, due to Drosophila-like
kinematics

Experiments
with "Robofly" showed that insects use three different aerodynamic
mechanisms to stay in the air.

They first
confirmed a previous theory, devised by a number of labs over the last
20 years, that a phenomenon called "delayed stall" occurs in the middle
of the stroke.

When the
insect sweeps its wing forward, a whirlpool or vortex of air is created
on top of the wings. This vortex seems to create a low-pressure zone that
produces lift.

The team
also discovered that two previously unknown forces occur at the end of
each half-stroke. When the wing rotates backward to change direction,
air is pulled over the top faster than the bottom, a force called "backspin."
Like a tennis ball with backspin, the wing is pulled upward by lower pressure.

In perhaps
the biggest surprise, another type of lift -- "wake capture" -- is also
created when a wing starts to change direction. The wing actually passes
through a spinning vortex wake from the previous stroke. Dickinson says
the wing can extract enough energy from this previous stroke to create
significant upward lift.

Dickinson
published these findings in the June 18, 1999 issue of Science.
But these discoveries are just the tip of the iceberg, Dickinson says.
Many questions are still unanswered. How
do flies turn? How do they navigate?